This invention relates to a mast assembly for attachment to a self propelled vehicle to constitute a forklift which is in widespread use for hoisting and moving heavy materials or objects. The invention is directed more specifically to a forklift mast assembly capable of sustaining nonperpendicular loading from its prongs to a greater extent than heretofore, with the consequent elimination of a tie or ties from between the masts and, accordingly, the enhancement of operator front vision.
Forklift mast assemblies in general have a pair of horizontally spaced apart outer masts and a pair of inner masts horizontally spaced apart to a less extent than the outer masts. The inner masts are arranged just inwardly of the outer masts for up and down motion along the same via a pair of outer rolls on the top ends of the outer masts and a pair of inner rolls on the bottom ends of the inner masts. For the up and down motion of the inner masts a pair of hydraulic cylinders are mounted upstandingly to the respective outer masts, with their piston rods arranged for extension and contraction with the inner masts. A pair of chains extend between the respective outer masts and a prong carrier means, having two or more lifting prongs affixed thereto, via respective sprockets that are movable up and down with the piston rods or with the inner masts. Thus the extension and contraction of the cylinders result in the up and down motion of the inner masts and of the prong carriers.
As heretofore constructed, however, the forklift mast assembly of the above general organization has had a problem arising from the cross sectional shapes of the outer and inner masts. Each outer mast has so far been of U shaped cross section, defining a space open toward the other outer mast to rotatably receive the inner roll on the bottom end of the associated inner mast. Each inner mast has also been of approximately U shaped cross section, defining a space open toward the other inner mast, but with a fin extending from its web toward the associated outer mast for rolling engagement with the outer roll on the top end of the outer mast. These cross sectional shapes of the outer and inner masts make it essential to interconnect both top and bottom ends of the outer masts, and of the inner masts, with ties to prevent them from spreading apart when loaded out of the perpendicular in its own plane from the prong carrier means. The mast ties have had to be of considerable thickness and width, with a sufficient strength to positively hold the masts in the correct relative working positions in the face of the nonperpendicular forces that may be exerted thereon from a considerable height from the bottom ends of the outer masts.
The presence of the bulky ties between the outer and inner masts seriously impairs the operator's front vision. An adequate perception of happenings to the front of the vehicle is essential for the proper operator control of the forklift. The mast ties are also objectionable as they render the mast assembly expensive, heavy, and difficult of assemblage.